Strings of light emitting diodes (LEDs), such as those used in advanced automotive headlights, also referred to as “matrix” headlights, use an LED driver to control the brightness of the headlight. Pulse Width Modulation (PWM) dimming is often used to control the light pattern and brightness of the LED string. Each LED in the LED string is configured to be controlled individually in order to control the overall light pattern and brightness of the headlight. To implement PWM dimming a boost stage is generally used to boost the voltage, resulting in a two-stage system.
A typical LED driver that uses PWM dimming for each individual LED is shown in FIG. 1. The LEDs (D112, D114, . . . D118, D120) are connected in series as an LED string 110. Although four LEDs are illustrated in FIG. 1, any number of LEDs can be provided in the LED string 110. There is a bypass switch (S112, S114, . . . S118, S120) in parallel with each LED (respectively, D112, D114, . . . D118, D120) controlled by a controller 130. If a switch is open (e.g., S112), the voltage across the corresponding LED (e.g., D112) is greater than its threshold voltage, and thus the current flows through the LED and it emits light. On the other hand, when the switch is closed, the voltage across the corresponding LED is less than the threshold so the LED is turned off. By selecting the relative duration of the on and off times of the switches, the average brightness of each LED may be individually controlled.